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Managing acute pain (Proceedings)

It is important to remember that pain is an experience, not a neurologic process. The experience of pain varies with many factors such as genetic make-up, sex, species and breed, hormonal influences, time of day, and age. Overall, there is about a 5-fold variation in the need for analgesics among normal individuals with similar injuries. In general, males require more analgesics than females, anxious individuals need more analgesics than calm individuals, and immature nervous system function is more deranged (and for longer periods) after injury than mature nervous system function.

Although acute pain has important survival benefit for untreated animals and man it has little benefit for hospitalized patients. When sufficiently intense, acute pain actually increases morbidity and mortality, primarily by excess amplification of the neuroendocrine response to injury. In hospitalized patients its value is greatly diminished; in fact, current ideology in human medicine holds that acute pain has no value at all in hospitalized patients, and every effort should be made to minimize it. This form of pain generally peaks in intensity within 24-72 hours of injury, and wanes progressively thereafter. In the absence of ongoing inflammation or repeated injury it is unusual for background pain (not necessary movement-associated) to be distressingly intense beyond 1-3 weeks. In fact, if debilitating pain remains beyond this time it usually indicates ongoing tissue pathology or a transition to chronic pain due to pathology within the nervous system.

This 3-week duration for acute pain overlaps with the same window of early opportunity for physical rehabilitation. Although movement-associated pain teaches the patient and caregivers what limits should be placed on physical therapy to avoid injury, left untreated it often interferes with reasonable goals for mobility and recovery of function. In some it is sufficiently intense to prevent adequate recovery altogether, setting the stage for abnormal neuromuscular function and chronic pain syndromes. Therefore, it is important to employ patient management strategies that maximize early recovery of function, particularly following orthopaedic procedures. Effective strategies often incorporate elements of both pharmacologic and physical therapy to achieve greater benefit than may be had by either alone.

The efficacy of acute pain control depends on how effectively you can interfere with its genesis at each step of nociception. From a drug therapy standpoint this interference should ideally be directed against nociception at the level of the first-order neuron to prevent any stimulation of higher-order nociception. The sensitivity of these neurons is enhanced by inflammation and can be modified with drugs such as nonsteroidal anti-inflammatory drugs. Signal transmission to the spinal cord can be completely blocked with local anesthetics. Physical maneuvers to modify inflammation (heat, cold, massage) will influence the response of the peripheral nervous system at this level. Nonsteroidal anti-inflammatory drugs, opioids, alpha-2 agonists, and NMDA receptor antagonists act in part by influencing processing and transmission at the level of the spinal cord. Physical therapy also affects spinal processing by modifying both nociceptive and somatosensory input. Stimulation of non-nociceptive input will not only interfere with acute pain (think of the effect of rubbing a painful elbow) but over time may interfere with the phenomenon of central sensitization.

In the worst stages of acute pain (for example the immediate postoperative period), drugs that alter consciousness and anxiety may modify pain without modifying nociceptive processing at the first two levels. For example, tranquilizers used at doses sufficient to relieve anxiety can promote restful REM sleep in the hours after exhausting surgery, limiting fatigue and its detrimental effects on pain and recovery. In man, patient education and incentive training are key elements to providing patients the psychological resources to master their pain, recover function, and minimize the risk of chronic pain or function loss. Although we are just beginning to understand the role of this in veterinary medicine there is abundant evidence attesting to the value of incentive, forced exercise, and placebo.